Clock circuit for a microprocessor

ABSTRACT

A mobile communication device includes an analog clock and a digital clock circuit. The analog clock circuit is configured to generate an oscillating output. The digital clock circuit is configured to generate a digital clock output having a frequency that is substantially equal to the frequency of the oscillating output.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 12/870,918, filedAug. 30, 2012, which is a continuation of U.S. application Ser. No.12/352,282, filed Jan. 12, 2009 (now U.S. Pat. No. 7,808,292), which isa continuation of U.S. application Ser. No. 11/446,915, filed Jun. 5,2006 (now U.S. Pat. No. 7,489,177), which is a continuation of U.S.application Ser. No. 10/744,793, filed Dec. 23, 2003 (now U.S. Pat. No.7,057,436), which is a continuation of U.S. application Ser. No.10/004,136, filed Oct. 22, 2001 (now U.S. Pat. No. 6,693,477), all theabove applications hereby incorporated by reference.

BACKGROUND

1. Technical Field

This relates to a clock circuit for microprocessors. It is particularlywell-suited for use in Personal Digital Assistants, mobile communicationdevices, cellular phones, and wireless two-way email communicationdevices (collectively referred to herein as “mobile devices”). It mayprovide utility, however, in any device that is subjected to high levelsof electromagnetic interference.

2. Description of the Related Art

Known clock circuits commonly include crystal oscillators that resonateat a certain frequency. Once the crystal oscillator begins vibrating atits resonant frequency, the resonant frequency is typically maintainedby feeding back an in-phase signal from one terminal of the crystaloscillator to the other terminal of the crystal oscillator. This allowsthe clock circuit to generate a substantially constant clock speed.

Known clock circuits generally have high impedances in order to reducepower consumption. The clock circuit's high impedance, however, makes itsusceptible to interference from strong electromagnetic signal sources.For example, in a mobile device, a transmission circuit may include apower amplifier that interferes with the clock circuit duringtransmission, temporarily changing the frequency of the oscillator.

Typical mobile devices isolate the clock circuit from the strongelectromagnetic interference that occurs during transmission byshielding the clock circuit with an EMI shield. In addition, a typicalmobile device may include an external buffer amplifier that protects theintegrity of the clock circuit. These known techniques generally addcomplexity and high component counts to the device. Furthermore, thesetechniques may require the circuit to restart the crystal oscillator inthe event that the oscillation has stopped or slowed as a result of astrong interference signal.

SUMMARY

A wireless communication device is described having a transmitter thatgenerates electromagnetic interference when operating in a transmitmode. The wireless communication device comprises a clock circuitincluding a first clock element configured to generate a first clockoutput, and a second clock element configured to generate a second clockoutput. Also included is a controller configured to switch between thefirst clock output and the second clock output when the transmitter isoperating in the transmit mode

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an exemplary clock circuit.

FIG. 2 is a timing diagram related to the circuit shown in FIG. 1.

DETAILED DESCRIPTION

With reference to the drawing figures, FIG. 1 is a circuit diagram of anexemplary clock circuit. The clock circuit includes a controller 12, ananalog clock element 21, a digital clock element 23, a state signal 14and a timing signal 16. The state signal 14 may, for example, be coupledto the transmission circuitry of a mobile device and configured todeliver a high pulse while the mobile device is transmitting. The timingsignal 16 may, for example, be generated by a frequency synthesizer in amobile device. The controller 12 is coupled to the timing signal 16 andthe state signal 14, and generates an analog clock enabling signal 20and a digital clock enabling signal 22. The analog clock enabling signal20 is coupled to the analog clock element 21, and the digital clockenabling signal 22 is coupled to the digital clock element 23. Therespective output terminals of the analog and digital clock elements 21and 23 are coupled together to generate the clock signal 10. Inaddition, the clock signal 10 is coupled as a trigger input 52 to thecontroller 12, and operates to synchronize the switching of the enablingsignals 20 and 22 such that the switching operation occurs timely toboth the period of the clock signal 10 and either a leading or fallingedge of the timing signal 16. The relationship between the trigger input52, the timing signal 16 and the enabling signals 20 and 22 is discussedbelow in more detail with reference to FIG. 2.

Operationally, the controller 12 enables either the analog clock element21 or the digital clock element 23 as a function of the state signal 14.For instance, while a mobile device is either idle or receiving atransmission, the state signal 14 may be in a first state, causing thecontroller 12 to enable the analog control element 21. Then, while themobile device is transmitting, the state signal 14 may be in a secondstate, causing the controller 12 to enable the digital clock element 23.In this manner, the clock circuit 14 isolates the analog clock element21 from the electromagnetic interference that is typically generatedwhile the mobile device is transmitting, and thus maintains asubstantially constant clock signal 10.

The analog clock element 21 preferably includes the oscillator amplifier26, a resistor 36, a crystal oscillator 40, and a plurality ofcapacitors 42, 44, and 46. The crystal oscillator 40, the oscillatoramplifier 26, and the resistor 36 are coupled in parallel. Thecapacitors 42 and 44 are coupled in series between ground and the inputof the oscillator amplifier 26. The capacitor 46 is coupled in seriesbetween the output of the oscillator amplifier 26 and ground. Theresistor 36 is a negative feedback element that couples the outputvoltage of the amplifier 26 to both the input of the crystal oscillator40 and the amplifier 26. In addition, the resistor 36 biases theamplifier 26 such that the amplifier 26 operates in a high-gain linearregion. Together, the amplifier 26 and the capacitors 44 and 46 shiftthe phase of the output to maintain oscillation. The values of theresistor 36 and the capacitors 44 and 46 are preferably determined bythe desired gain of the amplifier 26 and the load capacitance of thecrystal oscillator 40, respectively. In one alternative embodiment, theanalog clock circuit 21 may also include an additional resistor betweenthe output of the crystal oscillator 40 and output of the resistor 36.

The digital clock element 23 includes the digital amplifier 28 coupledto a frequency matching output 50 from the controller 12. The frequencymatching output 50 is preferably a square wave generated by thecontroller 12 that has a frequency substantially equal to the frequencyof the analog element 21. The frequency matching output 50 may begenerated, for example, by a divider internal to the controller 12 thatcounts cycles of the timing signal 16. In this manner, the frequencymatching output 50 is generated by dividing the frequency of the timingsignal 16. For instance, after a predetermined number of cycles of thetiming signal 16 have been counted, then the controller 12 can changethe signal level of the frequency matching output 50 from the low levelto the high level or, if the frequency output is high, from the highlevel to the low level. When the digital amplifier 28 is enabled by thedigital enabling signal 22, the digital amplifier 28 generates the clocksignal 10 with a frequency substantially equal to the frequency of thefrequency matching output 50.

Operationally, the state signal 14 determines which clock element 21 or23 generates the clock signal 10. When the state signal 14 is in a firststate, the oscillating amplifier 26 is enabled, the digital amplifier 28is disabled, and the clock signal 10 is generated by the analog clockelement 21. When the state signal 14 changes state, the controller 12preferably waits for a trigger input 52 and a new cycle in the timingsignal 16, and then reverses the states of the enabling signals 20 and22. Once the state change has been triggered, the digital amplifier 28is enabled, the oscillating amplifier 26 is disabled, and the clocksignal 10 is generated by the digital clock element 23. In addition, theclock signal 10 from the digital amplifier 28 is coupled to the crystaloscillator 40 through the feedback resistor 36 to enable the crystaloscillator 40 to continue to oscillate while the oscillating amplifier26 is disabled.

FIG. 2 is a timing diagram related to the circuit shown in FIG. 1. Thetiming diagram includes the state signal 14, the timing signal 16, ananalog output signal 58 (xouta), a digital output signal 60 (xoutb), andthe trigger input 52. The timing diagram also shows five dotted lines71, 72, 73, 74 and 75 that are included to refer to points in time whensignificant signal changes occur, and are respectively referred tohereinafter as REF 71, REF 72, REF 73, REF 74 and REF 75.

In a left to right view of the signals, the oscillator amplifier 26 isinitially enabled, generating an alternating high/low pulse as theanalog output signal 58. The digital amplifier 28 is initially disabled,with the digital output 60 in a high impedance state. The trigger input52, which is coupled both to the analog and digital output signals 58,is initially driven by the analog output signal 58.

At REF 71, the state signal 14 changes states (from low to high),instructing the controller 12 to switch from the analog clock element 21to the digital clock element 23. In order to synchronize the clockelement switch with the clock output 10, the controller 12 preferablywaits for the rising edge of the clock pulse 10 at its trigger input 52,indicating the beginning of a new cycle in the clock output 10. This isshown in FIG. 2 at REF 72. In addition, because the digital output 60 isgenerated as a function of the timing signal 16 as described above, thedigital and analog clock elements 21 and 23 should preferably not beswitched until the timing signal 16 starts a new cycle at REF 73.Otherwise, the clock output 10 could be switched over to the digitalclock element 23 before the controller begins dividing the timing signal16 to generate the frequency matching output 50, resulting in a glitchin the clock output 10. Thus, from REF 71 until REF 73 the clock output10 (and trigger input 52) is driven by the analog output 58. Thisresults in a short delayed pulse 80 from the analog output 58 thatoccurs before the clock elements 21 and 23 are switched at REF 73.

From REF 73 to REF 74, the digital amplifier 28 is enabled, generatingan alternating high/low pulse as the digital output signal 60 that has afrequency substantially equal to the oscillation frequency of theoscillator 40. The oscillator amplifier 26 is disabled, and its outputassumes a high impedance state. The trigger input 52 is being driven bythe digital output signal 60.

At REF 74, the state signal 14 again changes states (from high to low),instructing the controller 12 to switch back to the analog clock element21. As explained above, while the analog clock element 21 is disabled,the oscillator 40 is driven by the digital output signal 60 and thusmaintains synchronization with the clock output 10. Therefore, when thecontroller 12 receives a state signal 14 at REF 74, instructing a switchfrom the digital clock element 23 to the analog clock element 21, thecontroller 12 preferably only waits for an appropriate signal on itstrigger input 52 before initiating the switch. In a preferredembodiment, the switch between the digital output 60 and the analogoutput 58 is triggered at the falling edge of the trigger input signal52 (REF 75) to avoid glitches in the clock output 10.

In one exemplary embodiment, the timing circuit 8 described above withreference to FIGS. 1 and 2, may be implemented in a wirelesscommunication device. For example, a handheld, wireless communicationdevice may communicate with a base station through a wireless modem.When the handheld device is idle or is otherwise not communicating, thedevice sends a low state signal 14 to the circuit 8. The circuitsynchronizes the microprocessor of the device using the analog clockelement 21 which preferably resonates with a 32.768 kHz crystal. Duringthis time, the digital clock element 23 remains in a disabled state andthe digital amplifier output is in a high impedance state.

When the handheld device is communicating with the base station and thusgenerating a high level of electromagnetic interference, the devicesends a high state signal 14 to the circuit 8. The high state signal 14alerts the controller 12 to switch between the analog clock element 21and the digital clock element 23. When an edge of the analog outputsignal 58 is sensed at the trigger input 52, then the controllerdisables the analog element 21 and enables the digital element 23. Thecrystal is then protected from the high EMI fields associated withcommunication signals that may otherwise overwhelm the crystaloscillator 40 when the handheld device is transmitting voice and/or datamessages. Other electromagnetic disturbances which may similarly beanticipated by the device to initiate a switch from the analog clockelement 21 to the digital clock element 23.

The embodiments described herein are examples of structures, systems ormethods having elements corresponding to the elements of the claims.This written description may enable those skilled in the art to make anduse embodiments having alternative elements that likewise correspond tothe elements of the claims. The intended scope of the claims thusincludes other structures, systems or methods that do not differ fromthe literal language of the claims, and further includes otherstructures, systems or methods with insubstantial differences form theliteral language of the claims.

1. An electronic device comprising: a first clock circuit configured togenerate a first clock output at a first frequency; a second clockcircuit configured to generate a second clock output at a secondfrequency that is substantially equal to the first frequency; and acontroller configured to selectively enable only one of the clockoutputs at a time to provide a clock signal.
 2. The electronic device ofclaim 1 wherein the selectively enabling is by the controllerselectively enabling only one of the clock circuits at a time.
 3. Theelectronic device of claim 1 wherein the selectively enabling is by thecontroller enabling a selected one of the clock circuits when disablingthe other clock circuit.
 4. The electronic device of claim 1 wherein thecontroller has an input line into which both the first clock output andthe second clock output are directly fed.
 5. The electronic device ofclaim 1 wherein the selectively enabling is timed to coincide with arising or falling edge of a timing signal, the timing signal having afrequency greater than the frequency of the first clock output.
 6. Theelectronic device of claim 1 wherein the electronic device is a wirelesscommunication device, the first clock output is enabled as the clocksignal when the communication device is transmitting data, and thesecond clock output is enabled as the clock signal when thecommunication device is not transmitting.
 7. An electronic devicecomprising: a first clock circuit configured to generate a first clockoutput at a first frequency; a second clock circuit configured togenerate a second clock output at a second frequency that issubstantially equal to the first frequency; and a controller that has aninput line into which both the first clock output and the second clockoutput are directly fed.
 8. The electronic device of claim 7 wherein thecontroller is configured to selectively enable only one of the clockoutputs at a time to serve as a clock signal.
 9. The electronic deviceof claim 8 wherein the selectively enabling is by the controllerselectively enabling only one of the clock circuits at a time.
 10. Theelectronic device of claim 8 wherein the selectively enabling is by thecontroller enabling a selected one of the clock circuits while disablingthe other clock circuit.
 11. The electronic device of claim 8 whereinthe electronic device is a wireless communication device, the firstclock output is enabled as the clock signal when the communicationdevice is transmitting, and the second clock output is enabled as theclock signal when the communication device is not transmitting.
 12. Theelectronic device of claim 7 wherein the first and second clock circuitsare respectively an analog clock circuit and a digital clock circuit.13. The electronic device of claim 12 wherein the digital clock circuitis based on a counter whose input is a timing signal that has afrequency that is greater than the second frequency.
 14. An electronicdevice comprising: a first clock circuit configured to generate a firstclock output; a second clock circuit configured to generate a secondclock output; and a controller that has an input for inputting both thefirst clock output and the second clock output, and that is configuredto selectively enable only one of the clock circuits at a time.
 15. Theelectronic device of claim 14 wherein the selectively enabling is by thecontroller enabling a selected one of the clock circuits while disablingthe other clock circuit.
 16. The electronic device of claim 14 whereinthe selectively enabling is by the controller switching from the firstclock output being enabled as a clock signal to the second clock outputbeing enabled as the clock signal.
 17. The electronic device of claim 16wherein the controller is configured to wait for an edge of the firstclock output, indicating the beginning of a new cycle of the first clockoutput, before switching from the first clock output to the second clockoutput.
 18. The electronic device of claim 14 wherein the controller isconfigured to switch from one of the first clock output being enabled asa clock signal to the second clock output being enabled as the clocksignal in a manner that is synchronized with the first clock output. 19.The electronic device of claim 14 wherein the first and second clockcircuits are respectively an analog clock circuit and a digital clockcircuit.
 20. The electronic device of claim 14 wherein the electronicdevice is a wireless communication device, the first clock output isenabled as a clock signal when the communication device is transmitting,and the second clock output is enabled as the clock signal when thecommunication device is not transmitting.